This is a good trick to use when you want your site to be more likely to be indexed by search engines. Some robots don't follow links that have query strings on the end (ie, links with "?var=something"), so you can fool them by using the CGI environment variable PATH_INFO instead (however you choose to access it). You can also use it to make pages look half dynamic and half static for the search engines, to try and control what they index. For instance, if you've got a forum type script that has posts archived in the format "/forum/msgnum/", you can add bits at the end with the query string like "/forum/msgnum/?mode=threaded" or whatever (I think slashdot might do something like this, not entirely sure though).